In issue 3 the first of the three basic
circuits that can be built with the NE555 IC, the astable, was
explained. In this issue, we look at the monostable circuit which
can be used as part of a push-to-operate system for a
model at exhibitions.
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555 Timer Datasheet 141Kb |
When the monostable circuit is powered up, the NE555 Output (pin 3) will be 0V. It will remain at 0V indefinitely if the user does nothing. When a pulse is applied to the trigger input (pin 2), the Output goes high to Vs for a period of time called the time period. After this time the Output returns to the low, 0V, state. All of this is shown the timing diagram of figure 1.
Calculating the
time period
From the circuit diagram of figure
2, you can see that there is a resistor R and a
capacitor C which determine the time period.
The equation
relates the time period T in seconds to the capacitor value C in Farads and resistor value R in Ohms.
You can use the steps below to calculate the value of R. An example is shown in {curly brackets}:
Decide on the time period T which you require. This can be anything from milliseconds to several minutes, although you must express it in seconds. {I choose T=10s}
Next, guess a value for the capacitor C in farads. For starters, try 100mF. {I choose C=100mF}
Put the
values of T and C into the equation
below and calculate resistor R
If the value you get for R is not between 1kW and 1MW, select a different value for C and do the calculation again.
To reset the monostable circuit after it has been triggered, and before the time period has elapsed, you can momentarily connect the Reset pin to 0V.
An example stripboard layout for the circuit is shown in figure 12.
Need some help with these equations? Why not get the monostable wizard to do the work for you!
Only works with Microsoft Internet Explorer version 4.0 and
above.
See also: The NE555
astable circuit for an
explanation of the NE555 timer IC
Contrast with: The NE555
astable circuit , The NE555
bistable circuit
![]() |
Buy 555 Monostable Modules from the Electronics in Meccano Circuits Shop |
Here are a few extra techniques you can use with your NE555 circuits to make them easier to use and to add extra features
Sinking &
Sourcing
In the two circuits we have discussed so far, the component to be controlled, such as an LED, lamp or relay, has always been connected between pin 3 of the NE555 (its Output) and 0V. This is called sourcing and means that the component is switched on when the NE555 Output goes high.
If you instead connect the component between pin 3 and +Vs, it will be switched on when the NE555 is off. This technique is called sinking. It is possible to use both sinking and sourcing in the same circuit try the astable circuit from issue 3 with two LEDs connected to the output as shown in figure 3. The LEDs will flash on and off alternately. Remember to put in the correct LED resistors or, as you can see, there will be a direct current path from +Vs to 0V through the two LEDs and they wont like that very much!
Using a variable
resistor
To make an astable or monostable circuit with a variable frequency or time period, rather than fixed, you can replace one of the timing resistors (R, R1, or R2) with a variable resistor (also called a potentiometer or pot.)
There are two types of variable resistor linear and logarithmic. For this application the linear type (usually marked with LIN) is the most appropriate. The value marked on the variable resistor is its maximum resistance when the shaft is rotated fully, so you can still use the various equations to determine the lowest frequency or longest time period that the astable or monostable circuit will give. Because resistances in NE555 circuits should never be below 1kW, a 1kW resistor should be placed in series with the variable resistor.
In the astable circuit, you will find that replacing R1 with the variable resistor will vary the on time of the NE555, and replacing R with the variable resistor will vary both the on and off times of the NE555.
Standard variable resistors will have three connections the middle one (connected to the wiper) should be used along with any of the others. Variable resistors can be panel mounted (usually a 10.5mm hole) and have a plastic shaft which can be cut to the required length to fit a knob.
Other types of variable resistor
Preset resistors need to be PCB or stripboard mounted and are ideal for making infrequent adjustments to a circuit since a small screwdriver is needed to turn the shaft.
Switched types have a switch mechanism that is activated when the shaft is rotated fully to one extreme and can be used as an on/off switch for circuits.
Faders have a slider instead of a rotational shaft.
In electronics and science in general letter and symbol prefixes to units of measure are used to save having to write very large or very small numbers in full.
For example, the value 1,000,000W can be abbreviated to 1MW, the M meaning multiply by 1 million. In the same way, the value 0.000001F can be written as 1mF, the m meaning divide by 1 million.
In all the equations we have been discussing in the articles, the values you use must have their prefixes removed by multiplying/dividing as appropriate.
Below is a list of the prefixes you may come across, how you pronounce their names, and what you have to do to get the value without the prefix:
Prefix | Name | Meaning | To get value |
p | pico | (1 x 10-12) | Divide by 1 trillion |
n | nano | (1 x 10-9) | Divide by 1 billion |
m | micro | (1 x 10-6) | Divide by 1 million |
m | milli | (1 x 10-3) | Divide by 1 thousand |
k | kilo | (1 x 103) | Multiply by 1 thousand |
M | mega | (1 x 106) | Multiply by 1 million |
An Ohms Law example
Suppose you have a
3.3kW resistor with a current of 2mA passing through it
and you want to find out what voltage is across it.
You can use Ohms Law...
...but only after you have removed the prefixes from the values...
...and because every value was used in its basic unit, you can be sure that this answer is 6.6V.
Most of the circuits we have built in Electronics in Meccano need a smooth DC power supply. Some other circuits, such as those using logic ICs, also need their supply to be regulated.
Although you may have a mains transformer that steps-down the 230V AC supply (the UK mains power supply voltage), this may not have built-in smoothing or regulation.
Figure 5 shows a block diagram of the parts of a power supply system which turns a 230V AC mains supply into a regulated 5V DC supply. Assuming that you already have a suitable mains power supply with a transformer, such as those used to control model trains (I wouldnt recommend building your own because of the safety considerations), you will need to build the rectifier, smoothing and possibly the regulator blocks.
If the mains power supply that you start with does not have a cut-out or a fuse, add one of an appropriate rating as shown in the circuit diagram below.
What is AC and DC?
A representation of an AC
(Alternating Current) supply is shown on the right. The
voltage changes from positive to negative and back again
over time. In the case of the mains supply this will happen 50 times per second (50Hz). |
![]() |
![]() |
A DC (Direct Current) supply, shown left, stays at a fixed voltage all of the time (like the voltage from a battery). |
Parts of the power supply system
A full-wave
rectifier is made using four diodes wired up as shown in
the left-hand part of figure 11. A diode lets current pass in
only one direction through it, so the arrangement of the four
diodes means the positive parts of the wave-form are routed to
the +12V output and the negative parts are routed to the 0V
output.
Unfortunately, the output is not steady DC as it should be because the input voltage to the rectifier is always changing and sometimes it is zero, resulting in a waveform like the one in figure 8.
The output has to
be smoothed by placing a large value capacitor
across the output of the rectifier. The capacitor charges up as
the voltage rises and releases its energy slowly during the
periods when the supply voltage falls. The result is almost DC,
but there is still a small ripple voltage. Most
circuits can tolerate this, but some Integrated Circuits need the
supply voltage to be rock-steady.
To do this, a regulator IC is needed. These can take a range of DC voltages and produce a steady DC voltage. We will use the 78xx series of regulators which are wired up as shown on the right of figure 11. Choose the correct regulator for the voltage you require; 7805 for +5V (needed for TTL logic ICs), 7812 for +12V or 7815 for +15V.
The 78xx regulators can supply a current of
up to 1A, and there are of course other types of regulator that
deliver different voltages and currents.
![]() |
7805 Regulator Datasheet 72Kb |
Note that this is a very simplified explanation of power supplies since the output voltage of each stage (rectifier and smoothing) will not be exactly the same as the input voltage to that stage. Always check the output voltage before you connect the power supply to a circuit.
More on this in the next issue.
Continued in: More about
power supplies which gives
more details about smoothing and rectification.
![]() |
Buy 5V Regulated Power Supply Modules from the Electronics in Meccano Circuits Shop |
There are many
different types of connector that you can use in your models to
allow parts of the model or circuit to be easily dismantled and
assembled. A typical example would be in a crane, where the jib
may be removable, or a control panel that can be separated from a
model.
Single-way connectors
The following
types of connector allow only one wire to be connected. These are
generally used in applications where only a few connections are
needed (such as the two power supply connections to model), or
for connections that are frequently moved around (some designs of
control panel.)
Wander Plugs
Banana Plugs
In issue 5, Practical Matters will take a look at the different types of multi-way connectors such as DIN plugs, D plugs and PCB pin strips.
See also:
Practical Matters: Connectors for multi-way
connectors
The
following lists the electrical parts that are discussed in the
articles. Prices and order codes given are taken from the current
Maplin catalogue, which is the probably best source of electronic
components for the hobbyist in the UK.
If you have access to a company account with Rapid Electronics or RS Electronics you may find these companies are cheaper.
Also needed are suitable values for resistor R and capacitor C. |
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Maplin charge £2.50 for delivery on orders under
£30.00 inc. VAT.
Prices are taken from the September 2000 - August 2001 Maplin catalogue, and
include VAT at 17.5%
Contact their order line on 0870 264 6000 or visit one of their shops.
Their customer service line is 0870 264 6002 and
they have a website at www.maplin.co.uk where on-line ordering is
available.
www.eleinmec.freeserve.co.uk |
Electronics in Meccano June 1999 -- Issue 4 Edited by
Tim Surtell |
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